(1) Field of the Invention
The present invention relates to a light receiving device for reading optical media. More specifically, the present invention relates to a technique for accurately reading optical media compliant with different standards.
(2) Description of the Related Art
In recent years, optical media such as CDs (Compact Discs) and DVDs (Digital Versatile Discs) are widely used to store large amounts of music and video data, for example. In response to this trend, there is an increase in demand for optical pickups capable of reading both CD and DVD media. Unfortunately, however, the capability of handling both CD and DVD media often leads to the increase in the number of optical pickup parts or upsizing of the device.
To address the above problems, various optical pickups including the following have been suggested (see JP Patent Application Publication No. 2004-71058). FIG. 1 is a schematic view illustrating the structure of a conventional optical pickup. As illustrated in FIG. 1, an optical pickup 8 includes a monolithic integrated two-wavelength laser diode 801, a half-silvered mirror 802, a collimator lens 803, a mirror 804, an objective lens 805, a collective lens 806, and a light receiving element 807. The optical pickup 8 reads information from an optical medium 810.
The laser diode 801 emits laser light at different wavelengths depending on whether the optical medium 810 is a CD or DVD. The laser beam emitted form the laser diode 801 sequentially passes through the half-silvered mirror 802, the collimator lens 803, the mirror 804, and the objective lens 805, and converges at a point on the recording surface of the optical medium 810.
The laser light is then reflected off the recording surface of the optical medium 810 at a different intensity responsive to the state of the recording surface. The reflected light sequentially passes through the objective lens 805, the mirror 804, the collimator lens 803, the half-silvered mirror 802, and the collective lens 806 and is received by the light receiving element 807.
FIG. 2 is a plan view illustrating the structure of the light receiving element 807. As illustrated in FIG. 2, the light receiving element 807 includes ten photo diodes A1-A4, a1-a4, E and F. The photodiodes A1-A4 are provided to receive light reflected from CD, whereas photodiodes a1-a4 are provided to receive light reflected from DVD. The photodiodes E and F are provided to receive light reflected from either of CD and DVD.
The photodiodes A1-A4 and a1-a4 output radio frequency (RF) signals as well as focusing error signals. The photodiodes E and F output tracking error signals.
FIG. 3 is a circuit diagram illustrating the structure of a light receiving circuit including the light receiving element 807. As illustrated in FIG. 3, the light receiving circuit 10 includes differential amplifiers 1001-1004 each of which has a non-inverting input terminal and an inverting input terminal. Each non-inverting input terminal is applied with a reference voltage. Each inverting input terminal is connected in parallel to a different one of photodiode pairs, namely A1 & a1, A2 & a2, A3 & a3, and A4 & a4. With the structure stated above, each of the differential amplifiers 1001-1004 converts the current signal output from a corresponding one of the photodiode pairs A1 & a1, A2 & a2, A3 & a3, and A4 & a4 into a voltage signal and amplifies the resulting voltage signal.
In addition, the current signals output from the photodiodes E and F are also converted into voltage signals and amplified by the differential amplifiers 1006 and 1007, respectively. The output signals from the differential amplifiers 1001-1004 are added together by an adder 1005. The output signals of the differential amplifiers 1006 and 1007 are amplified by the amplifiers 1008 and 1009, respectively.
According to the structure stated above, the same optical parts are shared for handling both CD and DVD media. In addition, the same amplifier circuits are shared between paired photodiodes, one for CD and the other for DVD. Thus, the number of parts and the size the optical pickup are reduced.
It should be noted, however, that the optical pickup 8 illustrated in FIG. 3 involves the following setback. Since the inverting input terminal of each differential amplifier is connected to two photodiodes in parallel, dark current flowing through the two photodiodes is added together and amplified. Similarly, the influence of stray light doubles since the two photodiodes both receive stray light. The influence of electric current resulting from dark current and stray light (hereinafter, collectively referred to as “noise current”) is more notable after the noise current present in the two photodiodes is added by the adder 1005. The noise current inevitably reduces the reading accuracy of the optical pickup 8 and narrows the output dynamic range.